It has long been recognized that in the building of high quality bicycles, especially those used for competition, the frame must be accurately aligned. Specifically, the center lines of the main triangle of the frame, consisting of the top tube, seat tube and down tube, must describe a plane which coincides with the plane of the wheels (with the front wheel directed straight ahead) and the central movement, i.e., bottom bracket, must be precisely perpendicular to that plane. Bicycles which are not so aligned may present problems in handling and stability, and if the bottom bracket is not perpendicular to the plane of the main frame triangle, there may occur problems in the drive train and the derailleur system, and there may occur problems in the ability and ease with which the rider can pedal the bicycle.
The need for accurately checking the alignment of the bicycle frame occurs at numerous instances during the building of the frame, and also may occur in connection with later rebuilding or repair of the frame. During frame building, the down tube and seat tube must be joined to the bottom bracket shell, usually by a brazing process. The use of a jig to hold the bottom bracket shell, seat tube, and down tube during brazing will not ensure proper alignment, due to differential rate of expansion and distortion resulting from the application of heat. It is therefore necessary, as a practical matter, to check and recheck alignment at various steps throughout the building process to ensure accurate results.
In the prior art, almost all systems or tools for measuring any deviation of the seat tube and down tube from the perpendicular to the central movement rely on the faces, i.e., outermost edges, of the bottom bracket shell, to establish a line or plane which is then compared to that of the seat tube or down tube. In practice, the problem with this method is that the faces of the bottom bracket shell may not be exactly parallel to one another or perpendicular to the axis of the central movement. This situation can often result from imprecise or worn tools used to mill the faces of the bottom bracket shell. Since the distance between the two points on the bottom bracket faces which establish the reference line in such systems is only about 40mm, any small variances in the faces from one side of the bottom bracket shell to the other can result in large discrepancies when extended as references all the way along the seat tube or down tube. Obviously, it becomes impossible to accurately measure alignment under such circumstances.
Others have proposed the use of an alignment tool which relies on sleeves threaded into the bottom bracket shell and having a central bore into which a pin or rod is inserted, with a reference bar attached at right angles to the pin or rod. While theoretically overcoming the problem of non-parallel bottom bracket faces mentioned above, such systems in practice are subject to inacurracies caused by machining tolerances for the pin and bore or wear therein through use, with the result that the frame alignment reference provided by said tools is also subject to errors. Other systems have been proposed which use a combination of engagement of the bottom bracket faces and a bore in a sleeve threaded into the bottom bracket, but it is believed that such systems are subject to the same errors and inaccuracies as those discussed above.